1. Field of the Invention
The present invention relates to a portable semiconductor storage device and particularly, to a semiconductor storage device for replacing an electrochemical cell, that retains the stored data in the device, safely and easily.
2. Description of the Related Art
A conventional portable semiconductor storage device has a configuration such as that shown in FIG. 9. The storage device has a static RAM 1, an electrochemical cell 3, i.e., a battery, connected to the RAM 1 via an internal power-supply line 9 for supplying a power-supply voltage, a diode 5 for preventing reverse charging and a current limiting resistor 4. The cell 3 is held in a cell holder 3a which is removably provided in this storage device. The cell 3 is electrically connected by stationary electrode terminals 6 and 7 when the cell holder 3a is inserted into the storage device. A power-supply input line 8 is connected to the internal power-supply line 9 via a power-supply control circuit 2. The power-supply input line 8 is connected to a connector 10. An interface bus 11 and a cell monitor output line 12 are connected to the connector 10 from the RAM 1 and the stationary electrode terminal 6 on the positive electrode of the cell 3 respectively. The power-supply circuit 2 and the RAM 1 is connected by a control line 13.
A storage device thus configured is mounted on terminal equipment (not shown) via the connector 10 when in use. When a power-supply voltage above a prescribed value is applied to the power-supply input line 8, the power-supply control circuit 2 causes this power-supply input line 8 and the internal power-supply line 9 to be connected and sends out a high-level control signal to the RAM 1 via the control line 13. As a result, a power-supply voltage is supplied to the RAM 1 via the internal power-supply line 9. The RAM 1 is brought into an enable (operable) state by the high-level control signal. The terminal equipment can access the RAM 1, for example, reading from or writing into the RAM 1 via the interface bus 11.
When access to the RAM 1 is complete and a predetermined power-supply voltage is no longer applied to the power-supply input line 8, the power-supply control circuit 2 detects that the voltage value of the power-supply input line 8 has fallen below the prescribed value. The power-supply control circuit 2 disconnects the power-supply input line 8 and the internal power-supply line 9 and sends out a low-level control signal to the RAM 1 via the control line 13. The RAM 1 is brought into a disable (non-operable) state by this signal, and any access to it by the interface bus 11 is prohibited. At this time, the power-supply voltage from the power-supply input line 8 is not supplied to the RAM 1, but the voltage of the cell 3 is supplied to it via the resistor 4 and the diode 5. Therefore, if this storage device is removed from the terminal equipment, the stored data within the RAM 1 is not erased.
The cell 3 is gradually used up by supplying a voltage to the RAM 1. The voltage of the cell 3 is checked by the terminal equipment via the cell monitor output line 12 when the storage device is mounted on the terminal equipment. When the terminal voltage of the cell 3 falls below a predetermined voltage and the replacement of the cell 3 becomes necessary, the cell 3 can be removed from the storage device together with the cell holder 3a and replaced with a new one.
One example of the cell holder 3a is shown in FIG. 10. A projection 3b is formed on a side of the cell holder 3a. As shown in FIG. 11, when the cell holder 3a is inserted in the storage device, the projection 3b of the cell holder 3a fits into a recess 15 on a frame 14 of the storage device, holding the cell holder 3a in place. At this time, the stationary electrode terminals 6 and 7 contact the positive and negative electrodes of the cell 3, respectively. In FIG. 11, reference numeral 16 indicates a printed circuit board, on which each of the circuit elements shown in FIG. 9 is disposed.
However, if the cell holder 3a is removed to replace the cell 3 and a predetermined voltage is not continuously applied to the power-supply input line 8, the supply of power to the RAM 1 is interrupted and the stored data within the RAM 1 will be erased. Accordingly, the replacement of the cell 3 has to be made with the storage device mounted on the terminal equipment and a predetermined voltage applied to the power-supply input line 8 via the connector 10. As a result, there exists the problem in that replacement of cells is very inefficient and inconvenient.
Also, as electronic equipment become smaller and thinner, it is sometimes virtually impossible to replace a cell with the storage device mounted on terminal equipment.